Night vision devices enable a user to view a scene with little or no visible ambient light. Law enforcement and military personnel often use night vision devices during night time surveillance and maneuvers. Night vision devices typically employ an image intensifier tube that amplifies light in order to provide an enhanced image to the user.
Gated night vision devices, however, are susceptible to interference associated with oscillatory ambient light, which causes bright flashes of light that interfere with the viewed image. Some systems attempt to avoid the interference by determining the frequency of the ambient light and then using a gating frequency that avoids the interference. A disadvantage of these systems is that interference may occur if the frequency of the ambient light changes. Additionally, these systems may experience interference if the ambient light includes multiple frequencies or does not follow a regular pattern. Consequently, avoiding interference has posed challenges for the design of gated night vision devices and other gated sensors.
Gating the cathode of image tubes combined with periodic scene brightness variations can produce intermodulation products, which fall within the band that is detectable by the human eye. Since it is not possible to predict the frequency of scene brightness variations, such as line frequency flicker at 50, 60, 100, 120 hz, or computer monitor flicker at frequencies between 15 hz and several hundred khz, intermodulation products can be generated in the visible range.
Furthermore, gating at a fixed frequency may produce a gating signal, which occurs at a frequency that is a mechanical resonant frequency of image tube elements. This makes the image tube assembly produce an audible sound. Audible emissions from a night vision system are undesirable.